System and method for using a photovoltaic power source with a secondary coolant refrigeration system

ABSTRACT

A secondary coolant refrigeration system powered primarily by a photovoltaic source and by an alternating current (AC) source as a backup is disclosed. The secondary coolant refrigeration system has a pump for pumping secondary coolant fluid through a secondary coolant fluid loop. The system includes a variable frequency drive for controlling the speed of the pump. The variable frequency drive includes drive circuitry configured to provide variable frequency power to the pump via an output interface. The variable frequency drive also includes a first interface configured to receive power from the photovoltaic source and a second interface configured to receive power from the AC source. The circuit is further configured to cause the variable speed drive to be powered by the photovoltaic source when the power received from the first interface is adequate and by the AC source when the power received from the first interface is not adequate.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/131,457, filed Jun. 2, 2008, incorporated herein by reference in itsentirety.

BACKGROUND

The present disclosure relates generally to the field of refrigerationsystems. More specifically, the disclosure relates to systems andmethods for using a photovoltaic power source with a secondary coolantrefrigeration system.

It is well known to provide a refrigeration system including arefrigeration device or temperature controlled storage device such as arefrigerated case, refrigerator, freezer, etc. for use in commercial andindustrial applications involving the storage and/or display of objects,products and materials. For example, it is known to provide arefrigeration system with one or more refrigerated cases for display andstorage of frozen or refrigerated foods in a supermarket to maintain thefoods at a suitable temperature (e.g. 32 to 35 deg F). In suchapplications, such refrigeration systems often are expected to maintainthe temperature of a space within the refrigerated case where theobjects are contained within a particular range that is suitable for theparticular objects, typically well below the room or ambient airtemperature within the supermarket. Such known refrigeration systemswill typically include a heat exchanger in the form of a cooling elementor loop within the refrigeration device and provide a flow of a fluidsuch as a coolant into the cooling element to refrigerate (i.e. removeheat from) the space within the refrigeration device. Various knownconfigurations of refrigeration systems (e.g. direct expansion systemand/or secondary system, etc.) are used to provide a desired temperaturewithin a space in a refrigeration device such as a refrigerated case(e.g. by supply of coolant).

In refrigeration systems having a primary loop that circulates a directexpansion type refrigerant that interfaces with, and cools, a liquidcoolant in one or more secondary loop(s), the liquid coolant flowsthrough the secondary loops by way of one or more pumps, for examplemultiple variable speed pumps. The speed of the pump may be adjusted toprovide more or less pressure of the coolant in the coolant loops. Themotors for the one or more pumps for circulating the liquid coolantthough the secondary loops are conventionally powered by an electricalternating current (AC) source such as a power grid.

SUMMARY

One embodiment of the disclosure relates to a secondary coolantrefrigeration system powered primarily by a photovoltaic source and byan alternating current (AC) source as a backup. The secondary coolantrefrigeration system has a pump for pumping secondary coolant fluidthrough a secondary coolant fluid loop. The system includes a variablefrequency drive for controlling the speed of the pump. The variablefrequency drive includes drive circuitry configured to provide variablefrequency power to the pump via an output interface. The variablefrequency drive also includes a first interface configured to receivepower from the photovoltaic source and a second interface configured toreceive power from the AC source. The variable frequency drive furtherincludes a circuit configured to switch between providing power to thedrive circuitry from the first interface and providing power to thedrive circuitry from the second interface. The circuit is furtherconfigured to cause the variable speed drive to be powered by thephotovoltaic source when the power received from the first interface isadequate and by the AC source when the power received from the firstinterface is not adequate.

Another embodiment of the disclosure relates to a method for controllinga pump used in a secondary coolant refrigeration system. The speed ofthe pump is controlled using a variable frequency drive configured toselectively receive power from an AC source and a photovoltaic source.The method includes using the variable frequency drive to determinewhether power received from the photovoltaic source is adequate fordriving the pump. The method further includes configuring the variablefrequency drive to use the power received from the photovoltaic sourcewhen the power is adequate for driving the pump. The method yet furtherincludes using the variable frequency drive to switch from using thepower received from the photovoltaic source to power received from theAC source when the power received from the photovoltaic source isinadequate for driving the pump.

Another embodiment of the disclosure relates to a system for cooling aplurality of refrigeration loads. The system includes a plurality ofsecondary coolant fluid loops configured to cool the plurality ofrefrigeration loads. The system further includes a plurality of coolantpumps, each coolant pump associated with each secondary coolant fluidloop. The system yet further includes a plurality of variable frequencydrives, each variable frequency drive configured to controllably driveone of the plurality of coolant pumps and at least one photovoltaicpower source. The plurality of variable frequency drives are configuredto receive power from the photovoltaic power source and to use the powerfrom the photovoltaic power source to drive the coolant pumps when thepower from the photovoltaic power source meets a threshold requirement.Each variable frequency drive is also configured to receive power from asecond power source and to use the power received from the second powersource when the power from the photovoltaic power source does not meetthe threshold requirement.

Alternative exemplary embodiments relate to other features andcombinations of features as may be generally recited in the claims.

BRIEF DESCRIPTION OF THE FIGURES

The application will become more fully understood from the followingdetailed description, taken in conjunction with the accompanyingfigures, wherein like reference numerals refer to like elements, inwhich:

FIG. 1 is a block diagram of a refrigeration system utilizing asecondary coolant system, according to an exemplary embodiment;

FIG. 2A is a detailed block diagram of a refrigeration system andparticularly a variable frequency drive, according to an exemplaryembodiment;

FIG. 2B is a detailed block diagram of a refrigeration system andparticularly a variable frequency drive, according to another exemplaryembodiment; and

FIG. 3 is a flow chart of a process for controlling a pump used in asecondary coolant refrigeration system, according to an exemplaryembodiment.

DETAILED DESCRIPTION

Before turning to the figures, which illustrate the exemplaryembodiments in detail, it should be understood that the application isnot limited to the details or methodology set forth in the followingdescription or illustrated in the figures. It should also be understoodthat the phraseology and terminology employed herein is for the purposeof description only and should not be regarded as limiting.

According to any preferred embodiment, a secondary coolant refrigerationsystem, and more particularly a variable frequency drive for a pump ofthe system, is configured to be powered primarily by a photovoltaicpower source (e.g., a solar panel). The variable frequency drive is alsoconfigured to receive power from an AC source (or a secondary DC powersource). During operation, the variable frequency drive is configured toswitch from the photovoltaic power source to the AC source if the powerreceived from the photovoltaic power source (or available at the DCsource) is inadequate for driving the pump.

Referring to FIG. 1, a block diagram of a refrigeration system 100utilizing a secondary coolant system 101 is shown, according to anexemplary embodiment. Refrigeration system 100 is configured to providea cooling function to one or more refrigeration loads 104 by controllingcoolant flow through one or more secondary coolant fluid loops 102(e.g., a hydronic loop, a heat exchange loop, etc.), shown as fourloops, associated with refrigeration loads 104. Refrigeration loads 104may include any of a wide variety of objects to be cooled such astemperature controlled storage devices (e.g., refrigerated displaycases, walk-in coolers, etc.). Secondary cooling system 101 alsoincludes one or more pumps 106, one or more variable frequency drives(VFD) 108 associated with pumps 106, and a pump controller 110. Eachpump 106 is understood to include a motor that receives the AC electricpower from a VFD 108 and converts the electric power to rotationalmotion of a shaft which drives the pump.

Refrigeration system 100 is also shown to include a primary refrigerantloop 140 for circulating a refrigerant (e.g., a direct expansion typerefrigerant, etc.) through a compressor 142 and a condenser 144 and anexpansion device 146 to one or more chillers 148 and back to thecompressor 142. The chillers 148 are heat exchangers (e.g., plate typeheat exchangers or the like) shown to be located “downstream” of thesecondary coolant pumps 106 and to provide an interface between thesecondary coolant system 101 and the refrigerant of the primary loop 140to provide “chilling” or cooling of the secondary coolant fluid by therefrigerant.

According to one exemplary embodiment, refrigeration system 100 includesa secondary coolant system 101 with a plurality of branches or loops102, as may be used in refrigeration of refrigeration loads 104 such astemperature controlled storage devices in facilities such as foodretailing outlets (e.g., supermarkets, bakeries, etc.). According toother exemplary embodiments, refrigeration system 100 may be used withanother secondary coolant refrigeration system in any commercial,industrial, institutional or residential application or may include oneor more loops of a primary coolant refrigeration system. While FIG. 1illustrates four refrigeration loads 104 and a loop 102 associated witheach refrigeration load 104, according to other exemplary embodiments,there may be more or fewer than four loads and/or loops in the system.According to other exemplary embodiments, one loop may be associatedwith more than one load. According to still other exemplary embodiments,more than one loop may be associated with each load.

Pump 106 is configured to pump a coolant fluid through loops 102 ofsecondary coolant system 101 to provide cooling to refrigeration loads104. The coolant fluid may be any fluid capable of absorbing,transporting, and/or emitting heat (e.g., glycol, water, etc.). WhileFIG. 1 illustrates three pumps, according to other exemplaryembodiments, more or fewer than three pumps may be used. According tothe exemplary embodiment shown in FIG. 1, pumps 106 are variable speedalternating current (AC) electric motor pumps. Direct current (DC) pumpsmay be used according to various alternative embodiments. According toan exemplary embodiment, the pump is configured for use in secondarycoolant pump applications. Pump 106 may be a pump of any size suitablefor its intended application, but according to various exemplaryembodiments pump 106 has a horsepower range of 1-20 hp and a voltagerange of 208-575 volts AC.

VFD 108 (e.g., adjustable-frequency drive, variable-speed drive, ACdrive, microdrive or inverter drive, etc.) is a device configured tocontrol the rotational speed of a pump 106 by controlling the frequency(and thus voltage) of the electrical power supplied to pump 106. WhileFIG. 1 illustrates a VFD 108 corresponding to each pump 106, accordingto other exemplary embodiments one VFD may be used to control multiplepumps. According to various exemplary embodiments, the VFD may be asolid state device, for example using a rectifier bridge (e.g., diodebridge). According to other exemplary embodiments, the VFD may includeanalog circuitry. According to other exemplary embodiments, the VFD maybe another type of adjustable speed drive such a slip controlled driveor any other adjustable or variable speed drive.

Pump controller 110 is generally configured to control the fluid flow ofcoolant through system 101 based on pressure readings from loops 102.Pump controller 110 may control the fluid flow by controlling the speedof each individual pump 106, controlling the sequencing of the pumps,and/or conducting other pump controlling activities. According tovarious exemplary embodiments, pump controller 110 may be a digitaland/or analog circuit. According to other exemplary embodiments, pumpcontroller 110 may include a software controller executed on a processoror other circuit.

Referring still to FIG. 1, VFDs 108 are shown coupled to an AC powersource 112 and a photovoltaic power source 114. AC power source 112 maybe a power grid (e.g., 1-phase power, 3-phase power, 120-volt AC power,etc.). Photovoltaic power source 114 may be a solar cell, a solar array,a set of solar arrays, or any other photovoltaic modules. Photovoltaicpower source 114 may be any photovoltaic power source of the past,present, or future configured to receive solar energy and to output DCelectric power.

According to an exemplary embodiment, VFDs 108 are configured to use DCpower from photovoltaic power source 114 unless the received power isinadequate. In this event, VFDs 108 are configured to switch from usingthe DC power from photovoltaic power source 114 to using the AC powerfrom AC power source 112. According to various alternative embodiments,it is important to note that if power from photovoltaic power source 114is inadequate, VFDs 108 may be configured to switch to alternative powersources other than AC power source 112 (e.g., another DC power source, aback-up battery, one or more capacitors, etc).

Referring now to FIG. 2A, a close-up block diagram of a single VFD 108connected to power sources 112, 114 and pump motor 106 is shown,according to an exemplary embodiment. VFD 108 is shown to include an ACinterface 204, a DC interface 218, and a pump interface 212. VFD 108 isfurther shown to include a source switch 208, drive circuitry 210, acontroller 216, an input/output (I/O) terminal 224, and a controlinterface 226. According to an exemplary embodiment, the components ofVFD 108 are located together and are surrounded by a housing (e.g., aplastic housing, a metal housing, etc.).

Source switch 208 is configured to switch between different powersources such as AC power source 112 and photovoltaic power source 114.Source switch 208 may be implemented in a number of different ways. Forexample, a one or more diodes (or gates created using other electricalcomponents) may be used between the photovoltaic power source 114 andthe drive circuitry to allow or deny the flow of current fromphotovoltaic power source 114 to drive circuitry 210.

AC interface 204 and DC interface 218 may be or include any number ofjacks, terminals, receptacles, or other structures configured to receivecabling from power sources 112, 114. AC interface 204 and DC interface218 may also include circuitry configured to, for example, limit thecurrent received from the power sources, sense or detect the voltageavailable from the power sources, convert the AC to DC, invert the DC toAC, and/or to conduct other filtering, limiting, sensing, or convertingactivities on received power. Any number of protection or safetymechanisms such as diode 222 and/or switch 220 (e.g., fuse, circuitbreaker, etc.) may also or alternatively be provided between powersources 112, 114 and VFD 108. AC interface 204 is shown as connected toAC power source 112 by AC cabling 205. Similarly, pump interface 212 isshown as connected to pump motor 106 by AC cabling 213.

Drive circuitry 210 is configured to receive input from power bus 228,to controllably vary the frequency of the power, and to provide thepower to the output interface 212. Power bus 228 may be a DC power busand source switch 208 may include a converter that converts powerreceived from AC power source 112 to DC (if the photovoltaic powersource 114 is not being utilized). Drive circuitry 210 may include aninverter circuit with pulse width modulation voltage control, providingquasi-sinusoidal AC output. Drive circuitry 210 may include an embeddedmicroprocessor or may be controlled by controller 216.

Referring still to FIG. 2A, the decision regarding whether to switchfrom DC to AC and vice-versa may occur via controller 216. According toyet other exemplary embodiments, the decision regarding whether toswitch may be accomplished via a logic circuit that is a part of sourceswitch 208. Controller 216 may be a field programmable gate array(FPGA), an application specific integrated circuit (ASIC), a generalpurpose microcontroller configurable via computer code stored in memory,and/or any other combination of circuitry. Controller 216 and/or othercontrol circuitry of VFD 108 may be powered by battery, an auxiliary ACinput, AC power source 112 (even if photovoltaic power source 114 isbeing used to drive the pump, etc.). Controller 216 may be integratedwith source switch 208 (e.g., circuitry of source switch 208 and/or ofinterfaces 204, 218). According to an exemplary embodiment, controller216 makes a determination and controls a switch from photovoltaic powersource 114 to AC power source 112 in an automated fashion (e.g., doesnot require human input before each switch).

Controller 216 may be configured to conduct any number of activities todetermine whether or not to switch from DC to AC. Controller 216 may beconfigured to determine whether the power received from DC interface 218is adequate by comparing the power received at DC interface 218 to thepower received at AC interface 204. Controller 216, for example, may beconfigured to determine that the power received from DC interface 218 isadequate when voltage at DC interface 218 is equal to or greater thanthe voltage at AC interface 204. According to yet other exemplaryembodiments, controller 216 may be configured to determine that thepower received from DC interface 218 is adequate when voltage at DCinterface 218 is measured to be some multiple of voltage measured at ACinterface 204 (e.g., at least 1.35 times higher, etc.). According toother exemplary embodiments, controller 216 may be configured todetermine whether the power received from the DC interface 218 isadequate for driving pump 106 by comparing voltage from the DC interface218 to a threshold value (e.g., 400 VDC, etc.) or a setpoint of pump 106(e.g., as read and/or received from controller interface 226).

According to an exemplary embodiment, controller 216 is configured toseamlessly switch from using DC power to using AC power, and vice-versa.In other words, controller 216 may be configured to transition frompower source to power source in a manner that is transparent to endusers. Controller 216 may be configured so that manual input, userinput, or any other outside input (e.g., from pump controller 110 shownin FIG. 1) is not required for the switching to occur. A smoothtransition may be restored, for example, by receiving power from boththe AC power source 112 and the photovoltaic power source 114 for abrief period of time (e.g., input from AC source 112 is converted to DCand is provided in parallel to the DC from the photovoltaic power source114 to drive circuitry 210). According to other various exemplaryembodiments, the switch from DC to AC is timed so that the delay betweenswitching the DC off and providing the AC to the drive circuitry issmall (e.g., less than 100 milliseconds). According to various exemplaryembodiments, the DC to AC transition is accomplished at different ratesor via different methods but the transition is still smooth so as to noteffect the operation of the pump in a significant manner (e.g., notrequire the pump system to oscillate in order to re-obtain a lostsetpoint).

VFD 108 is further shown to include I/O terminal 224. I/O terminal 224may be or include one or more user controls built-into VFD 108. Forexample, I/O terminal 224 may include any number of buttons, keys,switches, potentiometers, displays, or the like for receiving tactileinput from a user. For example, VFD 108 may be configured to receiveinput from a user so that the user may adjust a threshold value used inthe determination of whether the DC power is sufficient. According tovarious other exemplary embodiments, I/O terminal 224 may be used towire VFD 108 to an external user input device (e.g., a terminal, akeyboard, etc.). Control signals may also be provided to VFD 108 (e.g.,from pump controller 110) via control interface 226. Control interface226 may include one or more jacks, terminals, receptacles, or otherstructures for receiving signals from another controller.

According to various exemplary embodiments, VFD 108 shown in FIG. 2A maybe configured so that power received from photovoltaic power source 114cannot be provided to AC power source 112 (i.e., VFD 108 does notinclude a “grid tied” inverter configured to provide power from thephotovoltaic power source 114 back to the power grid). In this and otherembodiments, VFD 108 may be configured or adjusted to accept a widerange of voltage levels from variously sized photovoltaic power sources(e.g., up to and/or greater than 800 VDC). According to an exemplaryembodiment, VFD 108 (and/or the photovoltaic power source 114) mayinclude components for converting excess power to heat and/or forstoring excess power received from photovoltaic power source 114 forlater use (e.g., in one or more coupled batteries or capacitor banks)Further, VFD 108 and/or pump 106 may be configured for regenerativebraking so that some kinetic energy of the system is stored and/orotherwise reused. For example, if a pump is commanded to slow down,kinetic energy from the pump system may be extracted from the pumpsystem, converted to electrical power, received by the variablefrequency drive, and stored in a battery system for later use.

According to an alternative exemplary embodiment, VFD 108 includes agrid tied inverter and is configured to provide power back to the powergrid in the event that the photovoltaic power source is providing morepower than the variable frequency drive needs to drive the pump.

Referring now to FIG. 2B, a diagram of VFD 258 connected to powersources 112, 114 and pump motor 106 is shown. According to the exemplaryembodiment shown in FIG. 2B, VFD 258's switch for transitioning from DCpower received via photovoltaic power source 114 and AC power source 112is the combination of rectifier bridge 260 and diode 262. Diode 262 isconfigured to have an “on-voltage” (i.e., “cut-in voltage”) that isabout 1.35 times the expected voltage from AC power source 112. Diode264 is a redundant diode used to protect the photovoltaic power source114 if diode 262 fails. It should be noted that the system will operatewith a diode in the position of diode 262, 264, or both. It shouldfurther be noted that a circuit different than diode 262 (and/or diode264) may be provided to VFD 258, the circuit configured to allow currentto flow to DC bus 228 when voltage from photovoltaic power source 114 issufficiently high. Referring further to the exemplary embodiment shownin FIG. 2B, when the voltage from photovoltaic power source 114 issufficiently high relative to voltage from rectifier bridge 260, diode262 allows current to flow from photovoltaic power source 114 to DC bus228. In this embodiment, rectifier bridge 260 is configured to impedethe flow of current from AC power source 112 to DC bus 228 when voltagefrom photovoltaic source 114 is greater than that available fromrectifier bridge 260 (e.g., the diodes of rectifier bridge 260 do notmeet their “cut-in voltage” due to the potential available on DC bus 228from photovoltaic power source 114 being larger than that available fromDC power source). According to various exemplary embodiments, the diodesof rectifier bridge 260 are sized so that they do no break down even ifphotovoltaic power source 114 is providing its maximum voltage.

In the exemplary embodiment illustrated in FIG. 2B, no active switchesare used to switch from photovoltaic power source 114 to AC power source112. Rather, the diodes (e.g., diode 262, the diodes of rectifier bridge260) are configured to behave similar to check valves. Current onlyflows in one direction, anode to cathode. The switching betweenphotovoltaic power source 114 and AC power source 112 is automaticallyconducted depending on the voltage of the sources and the properties ofthe diodes. According to an exemplary embodiment, the diodes areconfigured so that the following results occur: 1) if the voltage of thephotovoltaic source is greater than (1.35*voltage from the AC powersource 112), then current will flow from the photovoltaic source; 2) ifthe voltage of the photovoltaic source 114 is less than (1.35*voltagefrom the AC power source 112), then current will flow from AC powersource 112; 3) if the voltage of the photovoltaic source 114 is equal to(1.35*the voltage from the AC power source 112), then current will flowfrom both sources. According to an exemplary embodiment, if single phaseAC power source 112 is used, then the above equations will use 0.9rather than 1.35. According to an exemplary embodiment, thresholds of1.35*AC Voltage and 0.9*AC Voltage may change in different embodimentsdepending on the circuits and diodes used.

It is important to note that while three phase AC power, a rectifierbridge suitable for three phase AC power, and a three phase motor areshown in the drawings, a single phase motor, inverter, rectifier bridge,and/or AC power source may alternatively be used.

Referring now to FIG. 3, a flow chart of a process 300 for controlling apump used in a secondary coolant refrigeration system is shown,according to an exemplary embodiment. Process 300 is shown to includeconfiguring the VFD to use DC power from the photovoltaic power source(step 302). Step 302 may include any number of user input or adjustmentsteps. For example, step 302 may include selecting, providing, andconnecting the photovoltaic power source to the VFD. The photovoltaicpower source may be selected so that the output (e.g., output voltagerange) is within the range of acceptable DC input for the VFD. Dependingon the output characteristics of the photovoltaic power source, one ormore thresholds or bias settings of the VFD may be adjusted so that theVFD does not switch from DC to AC power too frequently. Further,computer code or other variables stored in memory of the VFD may beadjusted via one or more user interfaces.

Process 300 is further shown to include supplying DC power to the VFDvia the photovoltaic power source (step 304). Step 304 may include anumber of steps including a step of allowing current to flow (e.g., viaa gate, controlled diode, rectifier bridge, and/or switch) from the DCsource to the drive circuitry of the VFD. The VFD provides variablefrequency power to the pump motor (step 306) as long as the DC power issufficient for driving the pump (e.g., driving the pump though itsintended range of operation, driving the pump at the current setpoint,etc.).

While the variable frequency power is provided from the VFD to the pumpmotor, the VFD may be configured to check, measure, and/or determine(e.g., continuously, at an interval, etc.) whether the DC power providedby the photovoltaic power source is sufficient (step 308). Step 308 mayinclude any number of comparing steps (e.g., comparing the poweravailable from the DC power source to the power available from the ACpower source, comparing the power available from the DC power source toa threshold, determining if a pump setpoint is attainable and/ormaintainable using the DC power source, etc.).

If the DC power is sufficient, the VFD will continue to utilize powerfrom the photovoltaic power source (e.g., loop back to step 304). If DCpower is determined to be insufficient, the VFD is configured to begin aprocess of switching and/or transitioning to AC power (step 310).

The step of transitioning to AC power (step 310) may include any numberof shut-off, current limiting, disconnecting, and/or other techniquesrelative to the DC power source and any number of turn-on,current-allowing, connecting, and/or other techniques relative to the ACpower source. The step of transitioning (step 310) may be gradual,nearly immediate, or otherwise. Further, step 310 may also include anynumber of setting adjustments in memory of the VFD and/or via hardware.For example, the bias of a diode may be reversed, a different softwareroutine may be initiated, a variable in memory may be set, etc. After(and/or during) the transition to AC power, the power will be suppliedto the VFD via the AC power source (step 312). While AC power is beingutilized by the VFD, the VFD may regularly or continuously check forwhether the DC power has returned to a sufficient level for driving thepump (step 316). Program logic and/or circuitry within the VFD may beconfigured to immediately affect a switch to DC power if a threshold isreached. According to other exemplary embodiments, the switch may beaffected only if the threshold has been reached and maintained for aperiod of time (e.g., seconds, minutes, etc.). According to yet otherexemplary embodiments, program logic will examine other characteristicsof the DC power or the VFD (e.g., volatility, a standard of deviation,an average voltage measurement, a root mean squared measurement, thenumber of times switched to AC power during the day, the setpoint of thepump, a measurement of the associated refrigerator load, etc.) beforeswitching back from AC to DC. While DC power is determined to beinsufficient, the AC power source will continue supplying the drivingpower to the VFD (e.g., via a loop back to step 312, etc.).

It is important to note that the construction and arrangement of theelements of the refrigeration system are illustrative only. Althoughonly a few exemplary embodiments of the present disclosure have beendescribed in detail in this disclosure, those skilled in the art whoreview this disclosure will readily appreciate that many modificationsare possible in these embodiments (such as variations in features suchas components, formulations of coolant compositions, heat sources,orientation and configuration of the cooling elements, the location ofcomponents and sensors of the cooling system and control system;variations in sizes, structures, shapes, dimensions and proportions ofthe components of the system, use of materials, colors, combinations ofshapes, etc.) without materially departing from the novel teachings andadvantages of the disclosure. For example, closed or open spacerefrigeration devices may be used having either horizontal or verticalaccess openings; cooling elements may be provided in any number, size,orientation and arrangement to suit a particular refrigeration system;and the system may include a variable speed fan, under the control ofthe pump control system or otherwise.

Thresholds and/or set points of the controller or the switch may bedetermined empirically or predetermined based on operating assumptionsrelating to the intended use or application of the pump, variablefrequency drive, and/or the refrigeration devices. According to otheralternative embodiments, the refrigeration system may be any deviceusing a refrigerant or coolant, or a combination of a refrigerant and acoolant, for transferring heat from one space to be cooled to anotherspace or source designed to receive the rejected heat and may includecommercial, institutional or residential refrigeration systems. Further,it is readily apparent that variations of the refrigeration system andits components and elements may be provided in a wide variety of types,shapes, sizes and performance characteristics, or provided in locationsexternal or partially external to the refrigeration system. Accordingly,all such modifications are intended to be within the scope of thedisclosure.

It should further be noted that the variable frequency drive describedherein and the switching activity from a photovoltaic power source to anAC power source may be applicable in some applications other than thesecondary coolant refrigeration application.

Although the figures may show a specific order of method steps, theorder of the steps may differ from what is depicted. Also two or moresteps may be performed concurrently or with partial concurrence. Suchvariation will depend on the software and hardware systems chosen and ondesigner choice. All such variations are within the scope of thedisclosure. Likewise, software implementations could be accomplishedwith standard programming techniques with rule based logic and otherlogic to accomplish the various connection steps, processing steps,comparison steps and decision steps.

Embodiments within the scope of the present disclosure include programproducts comprising machine-readable media for carrying or havingmachine-executable instructions or data structures stored thereon (e.g.,program products/software for controlling variable frequency drives).Such machine-readable media can be any available media that can beaccessed by a general purpose or special purpose computer or othermachine with a processor. By way of example, such machine-readable mediacan comprise RAM, ROM, EPROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage or other magnetic storage devices, or anyother medium which can be used to carry or store desired program code inthe form of machine-executable instructions or data structures and whichcan be accessed by a general purpose or special purpose computer orother machine with a processor. When information is transferred orprovided over a network or another communications connection (eitherhardwired, wireless, or a combination of hardwired or wireless) to amachine, the machine properly views the connection as a machine-readablemedium. Thus, any such connection is properly termed a machine-readablemedium. Combinations of the above are also included within the scope ofmachine-readable media. Machine-executable instructions comprise, forexample, instructions and data which cause a general purpose computer,special purpose computer, or special purpose processing machines toperform a certain function or group of functions.

What is claimed is:
 1. A refrigeration system powered primarily by aphotovoltaic source and by an alternating current (AC) source as abackup, the refrigeration system having a primary loop and a secondaryloop, the refrigeration system having a pump for pumping coolant fluidthrough the secondary loop, the system comprising: a variable frequencydrive for controlling the speed of the pump, the variable frequencydrive comprising: drive circuitry configured to provide variablefrequency power to the pump via an output interface; a first interfaceconfigured to receive power from the photovoltaic source; a secondinterface configured to receive AC power from the AC source; a DC buscoupled to the drive circuitry and configured to transmit power to thedrive circuitry; a rectifier coupled to the second interface and to theDC bus, wherein the rectifier is configured to receive the AC power fromthe AC source, convert the AC power into DC power, and transmit the DCpower to the drive circuitry via the DC bus; and a diode coupled to thefirst interface and to the DC bus, wherein the diode is configured toreceive the power from the photovoltaic source and transmit the power tothe drive circuitry via the DC bus, wherein the rectifier and the diodeare configured to switch between providing power to the drive circuitryfrom the first interface and providing power to the drive circuitry fromthe second interface, and wherein the rectifier and the diode areconfigured to cause the drive circuitry to be powered by power receivedat the first interface when the power from the photovoltaic source isadequate to power the drive circuitry and to be powered by powerreceived at the second interface when the power from the photovoltaicsource is not adequate to power the drive circuitry.
 2. Therefrigeration system of claim 1, wherein the rectifier and the diode areconfigured to cause the switch between the first interface and thesecond interface to be smooth so that operation of the pump is notinterrupted.
 3. The refrigeration system of claim 1, wherein theswitching is automated and does not require human input before eachswitch.
 4. The refrigeration system of claim 1, wherein the rectifierand the diode are configured to determine whether the power receivedfrom the first interface is adequate by comparing the power received atthe first interface to the power received at the second interface. 5.The refrigeration system of claim 1, wherein the rectifier and the diodeare configured to determine that the power received from the firstinterface is adequate when voltage at the first interface is equal to orgreater than the voltage at the second interface.
 6. The refrigerationsystem of claim 1, wherein the rectifier and the diode are configured todetermine that the power received from the first interface is adequatewhen voltage at the first interface is at least 1.35 times higher thanthe voltage at the second interface.
 7. The refrigeration system ofclaim 1, wherein the rectifier and the diode determine whether the powerreceived from the first interface is adequate to power the drivecircuitry by comparing voltage from the first interface to a thresholdvalue.
 8. The refrigeration system of claim 7, wherein the variablefrequency drive further comprises an input for receiving a signal from auser interface and wherein the controller is further configured toadjust the threshold value based on the received signal.
 9. Therefrigeration system of claim 1, further comprising: wherein the pump isone of a plurality of pumps for pumping coolant fluid; and wherein thevariable frequency drive is one of a plurality of variable frequencydrives, each variable frequency drive configured to drive one of theplurality of pumps, wherein each variable frequency drive is configuredto receive power from the photovoltaic source or another photovoltaicsource by default.
 10. The secondary coolant refrigeration system ofclaim 1, wherein the circuit comprises a diode connected to thephotovoltaic power source, the diode configured to have an on-voltage ofat least the normal output voltage from the AC source.
 11. The secondarycoolant refrigeration system of claim 10, wherein the circuit furthercomprises a rectifier configured to receive power from the secondinterface and to provide power to a direct current (DC) bus, the DC buscouples the second interface to inverter circuitry of the variablefrequency drive, and wherein the output from the cathode of the diode isconnected to the DC bus.
 12. The refrigeration system of claim 1,further comprising a programmable controller configured to controlwhether power is received by the drive circuitry from the firstinterface or the second interface.
 13. A method for controlling a pumpused in a secondary coolant refrigeration system, the speed of the pumpcontrolled using a variable frequency drive configured to selectivelyreceive power from an AC source and a photovoltaic source, the methodcomprising: using the variable frequency drive to determine whetherpower received from the photovoltaic source is adequate for driving thepump; configuring the variable frequency drive to use the power receivedfrom the photovoltaic source when the power is adequate for driving thepump; and using the variable frequency drive to switch from using thepower received from the photovoltaic source to power received from theAC source when the power received from the photovoltaic source isinadequate for driving the pump.
 14. The method of claim 13, wherein thevariable frequency drive is configured to use power from thephotovoltaic source when the voltage available from the photovoltaicsource is equal to or greater than the voltage available at the ACsource.
 15. The method of claim 13, wherein the variable frequency driveis configured to use power from the photovoltaic source when the voltageavailable from the from the photovoltaic power source at least 1.35times higher than the voltage available at the AC source when the ACsource is a three phase AC source and at least 0.9 times higher than thevoltage available at the AC source when the AC source is a single phaseAC source.
 16. The method of claim 13, further comprising: comparing ameasurement of voltage of the power received from the photovoltaicsource to a threshold value.
 17. The method of claim 16, furthercomprising: receiving an input from a user interface; and adjusting thethreshold value based on the received input.
 18. The method of claim 13,further comprising: configuring the variable frequency drive to receivepower from the photovoltaic source by default and the AC source as abackup.
 19. The method of claim 13, further comprising: comparing thepower received from the photovoltaic source to the power available fromthe AC source.
 20. A system for cooling a plurality of refrigerationloads, the system comprising: a plurality of secondary coolant fluidloops configured to cool the plurality of refrigeration loads; aplurality of coolant pumps, each coolant pump associated with at leastone secondary coolant fluid loop; a plurality of variable frequencydrives, each variable frequency drive configured to control and driveone of the plurality of coolant pumps; and at least one photovoltaicpower source; wherein the plurality of variable frequency drives areconfigured to receive power from the photovoltaic power source and touse the power from the photovoltaic power source to drive the coolantpumps when the power from the photovoltaic power source meets athreshold requirement, and wherein each variable frequency drive is alsoconfigured to receive power from a second power source and to use thepower received from the second power source when the power from thephotovoltaic power source does not meet the threshold requirement,wherein the threshold requirement is a minimum voltage requirement andwherein the second power source is one of grid power and a batterybackup source, wherein a separate photovoltaic power source is providedfor each of the variable frequency drives.